Clearly a stance violation based on 802.07.A.2- "Have no supporting point closer to the target than the rear edge of the marker disc"
Not sure how it could be construed to be an equipment violation.
This is really "down in the weeds" kind of question, and I think the way the rule should read so that it would clearly would just be a stance violation. It's clearly some sort of violation.
The nitpicking question at hand is whether the marker disc becomes part of the playing surface when it's on the ground. If it does, then it's a stance violation. If it doesn't then … you don't have a supporting point past the lie if you have your foot only partially on the disc. (This is different than simply bumping it forward, which is clearly a stance violation.)
The illegal device comes in here:
813.02 Illegal Device
D. An item such as a towel or a pad may be placed under a supporting point as long as it is not greater than one centimeter in thickness when compressed.
The disc is an item. It doesn't compress to one centimeter. It's illegal.
Example of how this could possibly even "matter" (in quotes because I acknowledge this is a very fringe case):
Two discs have come to rest in some mud, one right behind the other. Player A should throw first, but player B asks if he can throw first and the group says yes. Player B proceeds to stand on Player A's disc in order to avoid sinking into the mud (let's assume Player A has no objection to this for the hypothetical).
If the disc is actually part of the playing surface, there is no violation.